1. Field of the Invention
The present invention relates to an improved tubular structure with unique properties suitable for a wide array of applications, including use in manufacturing, as apparatus for positioning and deploying medical diagnostic and treatment devices in a body and in other uses.
2. Discussion of the Related Art
It is a known property of many tubular constructs, such as those made from flexible plastic materials, that the tube will contract in diameter if the tube is elongated longitudinally. This property is commonly referred to as “necking.” Such necking can be problematic in many applications.
For instance, if a plastic wrap is applied over a mandrel in a manufacturing process, pulling on the end of the plastic wrap to remove the wrap from the mandrel will result in the wrap necking down on the mandrel. This often makes it difficult or impossible to slide the plastic wrap off the mandrel, requiring cutting of the wrap or distortion of the mandrel to separate the wrap.
Similarly, necking can likewise be a factor if a plastic tube is used to contain or constrain a device. For instance, in self-expanding medical devices for remote deployment in a patient, such as a stent or blood filter, the device designer must accommodate necking of the plastic tube if it is to be separated from the medical device through the relative sliding of the device and the constraining tube. Typically this requires using plastic tubes that resist necking, such as thicker and/or stiffer materials that can add undesirable profile to the device and/or reduce its flexibility and maneuverability within the body. Optimizing compactness and flexibility are highly desirable as physicians try to reach tighter treatment sites through smaller and more tortuous vessels.
Alternatively, a medical device designer may employ other deployment methods to separate the tube from the implantable device. For instance, a constraining sleeve can be designed to be cut or split from the implantable device, such as is described in U.S. Pat. No. 6,352,561 to Leopold et al. Others have suggested everting the sleeve to reduce the force required to slide the sleeve from the implantable device. Variations on this concept are described in, for instance, U.S. Pat. No. 4,732,152 to Wallsten, U.S. Pat. No. 5,571,135 to Fraser et al., U.S. Pat. No. 6,942,682 to Vrba et al., US Application 2006/0025844 to Majercak et al., and US Patent Application 2006/0030923 to Gunderson.
While everting sheaths can reduce the tension that must be applied to the constraining sleeve, they still can require considerable tension in order to pull the sheath over itself and the self-expanding device during deployment, resulting mainly from the friction of everted portion of the sheath rubbing against the non-everted portion of the sheath while the sheath is being removed. To whatever degree the sleeve material necks down on the device during deployment, this further complicates device design. These concerns are compounded with longer device lengths and more tightly compacted self-expanding devices that exert greater outward pressures. The greater the tension needed to evert and remove the sheath, the more demanding it is for the medical staff to remove the sheath while trying to hold the apparatus in its exact position during deployment. Increased deployment tensions also require more substantial sheath constructions so as to avoid sheath and deployment line breakage during deployment. It is believed that these deficiencies of everting sheaths may have limited practical applications for such deployment methods.
In co-pending U.S. Application Publication 2009/0182411 (Ser. No. 12/014,536 to Irwin et al.) a deployment sheath has been proposed that includes diametrically stored material to assist in sheath removal during implantable device deployment. For example, by constructing a constraining sheath with one or more folds or “pleats,” it is much easier to evert the sleeve over itself during deployment by allowing the pleats to open as the sheath everts over itself. This essentially produces an inverse effect from necking—as the pleats open, the tubular sheath appears to grow diametrically while it is everting on itself. It has been found that this greatly aids in the deployment process. As a result, such pleated deployment sheaths are believed to be useful in a wide array of medical diagnostic and treatment devices, including stents, stent-grafts, balloons, blood filters, occluders, probes, valves, electronic leads, orthopedic devices, etc.
Plainly pleated tubes can be used not only to address the problem of necking, but actually can allow the tube to increase in effective diameter as axial force is applied to the tube. This is a major advancement over prior medical device deployment apparatus. Nevertheless, providing pleated sheaths with tightly controlled “growth” properties, as is required for medical device deployment, requires careful design and quality assurance controls. Pleated sheaths also work best when deployed in everted tube configurations.
Employing a pleated sheath to constrain and deploy a medical device such as when used as described above, may be desirable but it is recognized that a single layer of material would be preferable for such applications since it further reduces device profile.
Accordingly, it would be desirable to develop a tubular apparatus that is capable of increasing diametrically as axial tension is applied to it.
It would be further desirable to develop such a tubular construct that increases in diameter when axially elongated that can be used in single or multiple layers, both with and without pleats.
In addition, some stent or stent-graft designs include biased helical frames that are sheath constrained and foreshorten during deployment, making accurate placement difficult. The most common method of deploying a stent or stent-graft, as described herein, involves a removable constraining sheath. Sheath removal is accomplished through application of an axial force that is transmitted through the length of the catheter. Disadvantages associated with sheath constraint and deployment, in general, include sheath removal forces that scale with device length, packing pressure, compounding forces due to buckling of axially stressed catheter elements, catheter strain and added profile to the sheath itself. Although some of these issues are addressed herein, there are still disadvantages to using sheaths, currently known in the art, for constraining and deploying stents and other devices.